Most electronic components, such as transistors and diodes, generate large amounts of thermal energy, heat, during use. This heat must be removed from the area of the electronic device as it is well known that a buildup of heat adversely effects the performance characteristics of electronic components.
The use of a heat sink is one method of removing heat from such components. The heat sink draws the heat away from the device via conduction and dissipates the heat by convection, radiation or both. However, the interface between the heat sink and the device is not perfectly smooth. Irregularities in the heat sink and/or device surface, even on a microscopic scale, form pockets in which air can become entrapped. These pockets reduce the efficiency of the thermal transfer in that there is less surface contact between the heat sink and component through which heat can be conducted. Further, it is well known that air is a poor thermal conductor, therefore the presence of air pockets drastically reduces the rate of thermal transfer between the heat sink and device. To counter this problem, it is known to use a layer of thermally conductive material between the heat sink and device to fill in the surface irregularities and eliminate air pockets.
Perhaps the best known and widely used material is a semi-liquid silicone grease. The grease is generally filled with a thermally conductive filler, such as aluminum oxide, to enhance its conductive properties.
Alternatively, filled waxes have been suggested as a conductive material. See U.S. Pat. No. 4,299,715.
Both the greases and waxes are messy to form and use, are generally in a flowable state at the operating temperature of the device, and are subject to migration. The migration of these materials is difficult, if not impossible, to control and the greases and waxes contaminate contacts, switches and leads making them difficult to solder and in some cases causing them to cease functioning.
Containment of the silicone grease and the wax have been suggested, but the success of such efforts has been minimal. See U.S. Pat. Nos. 3,391,242 and 4,466,483.
Another approach used to increase the heat transfer between the heat sink and electronic device is to use a filled silicone rubber sheetlike material instead of the silicone grease or wax. The silicone rubber in general is sufficiently soft to conform to the surface irregularities existing on the two interfacing surfaces of the heat sink and the device. The addition of a thermally conductive filler enhances the silicone's thermal transfer abilities to a level comparable to that of the greases and waxes.
The silicone rubber materials have been an improvement over the greases and waxes in that they tend to be easier to use and apply. The silicone rubber materials however have several drawbacks which make them less than ideal candidates. Silicone rubbers exhibit the same migration problem as the greases and waxes. The rubbers tend to spread and flow when subjected to pressure and/or heat. This tendency causes the rubber to flow to areas where its presence is not desired, often resulting in contaminated switches and leads which are either rendered unsolderable or inoperable or both. Additionally, silicone rubbers resist coating by other materials. This tendency is particularly troublesome in the electronic industry where conformal coatings are widely used. The term "conformal coatings" as used herein is defined as thin layers of protective materials which are applied to circuit boards and other electronic components to prevent oxidation, water or dust damage, or contamination.
The present invention provides a novel thermally conductive material which is resistant to migration and contamination and is able to accept conformal coatings.